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Fix outdated LFU comments to eliminate confusion (#12244)

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The decrement time was replaced by access time in
583c31472577fb8175e17ee0ce243972f4dd8425.

The halved and doubled LFU_INIT_VAL logic has been changed in
06ca9d683920da19ad53532f8cd55b54584027bc.
Now we just decrement the counter by num_periods. This has been
previously fixed in redis.conf, #11108.
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Binbin 2023-12-07 02:46:57 +08:00 committed by GitHub
parent 77e69d8884
commit 2f6d4dabaa
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1 changed files with 13 additions and 15 deletions
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src/evict.c
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@ -252,42 +252,40 @@ int evictionPoolPopulate(int dbid, int slot, dict *sampledict, dict *keydict, st
*
* We split the 24 bits into two fields:
*
* 16 bits 8 bits
* +----------------+--------+
* + Last decr time | LOG_C |
* +----------------+--------+
* 16 bits 8 bits
* +------------------+--------+
* + Last access time | LOG_C |
* +------------------+--------+
*
* LOG_C is a logarithmic counter that provides an indication of the access
* frequency. However this field must also be decremented otherwise what used
* to be a frequently accessed key in the past, will remain ranked like that
* forever, while we want the algorithm to adapt to access pattern changes.
*
* So the remaining 16 bits are used in order to store the "decrement time",
* So the remaining 16 bits are used in order to store the "access time",
* a reduced-precision Unix time (we take 16 bits of the time converted
* in minutes since we don't care about wrapping around) where the LOG_C
* counter is halved if it has an high value, or just decremented if it
* has a low value.
* counter decays every minute by default (depends on lfu-decay-time).
*
* New keys don't start at zero, in order to have the ability to collect
* some accesses before being trashed away, so they start at LFU_INIT_VAL.
* The logarithmic increment performed on LOG_C takes care of LFU_INIT_VAL
* when incrementing the key, so that keys starting at LFU_INIT_VAL
* (or having a smaller value) have a very high chance of being incremented
* on access.
* on access. (The chance depends on counter and lfu-log-factor.)
*
* During decrement, the value of the logarithmic counter is halved if
* its current value is greater than two times the LFU_INIT_VAL, otherwise
* it is just decremented by one.
* During decrement, the value of the logarithmic counter is decremented by
* one when lfu-decay-time minutes elapsed.
* --------------------------------------------------------------------------*/
/* Return the current time in minutes, just taking the least significant
* 16 bits. The returned time is suitable to be stored as LDT (last decrement
* 16 bits. The returned time is suitable to be stored as LDT (last access
* time) for the LFU implementation. */
unsigned long LFUGetTimeInMinutes(void) {
return (server.unixtime/60) & 65535;
}
/* Given an object last access time, compute the minimum number of minutes
/* Given an object ldt (last access time), compute the minimum number of minutes
* that elapsed since the last access. Handle overflow (ldt greater than
* the current 16 bits minutes time) considering the time as wrapping
* exactly once. */
@ -309,10 +307,10 @@ uint8_t LFULogIncr(uint8_t counter) {
return counter;
}
/* If the object decrement time is reached decrement the LFU counter but
/* If the object's ldt (last access time) is reached, decrement the LFU counter but
* do not update LFU fields of the object, we update the access time
* and counter in an explicit way when the object is really accessed.
* And we will times halve the counter according to the times of
* And we will decrement the counter according to the times of
* elapsed time than server.lfu_decay_time.
* Return the object frequency counter.
*